DescriptionThiourea organocatalysis represents a versatile activation strategy in synthetic organic chemistry. By hydrogen-bonding interactions with neutral or anionic substrates and/or intermediates, thiourea organocatalysts can often direct the reaction pathways with promising levels of stereochemical control. This dissertation demonstrates the cooperative use of thiourea organocatalysts in combination with other types of catalysts in addressing challenging synthetic problems that are not realized by either type alone. Specifically, cooperative catalysis of thiourea and iminium organocatalysts enabled the first highly enantioselective direct oxa-Pictet–Spengler reactions under weakly acidic conditions. In comTbination with a transition metal catalyst, an acid–thiourea catalyst afforded highly enantioselective A3 reactions with secondary amines. Strong Brønsted acid was also applied cooperatively with thiourea organocatalyst, furnishing the direct reductive etherification reactions with a wide variety of substrates.